Wow, direct reply !

Maybe my question was stupid ?
I just folow the link you put on your post, and i read it:

iconCls : String A CSS class that will provide a background image to be used as the header icon (defaults to ''). An example custom ic...
A CSS class that will provide a background image to be used as the header icon (defaults to ''). An example custom icon class would be something like: .my-icon { background: url(../images/my-icon.gif) 0 6px no-repeat !important;}

So, am I stupid to think that extjs was able to setting new css ???

I should use ext.util.css ?

A class is a class name. A DOM Element has a string which can contain class names.
You then specify styling rules which use selectors which are based on class names to apply the rules to elements.